1. Field of the Invention:
This invention relates to a semiconductor laser apparatus in which a laser-oscillating area and a light-detecting area are incorporated into one body.
2. Description of the prior art:
FIG. 2 shows a conventional semiconductor laser apparatus 1, which is produced as follows: On an n-InP substrate 2, an n-InP buffer layer 3, an InGaAsP active layer 4, and a p-InP cladding layer 5 are successively formed by liquid phase epitaxy. A common electrode 6 is formed on the back face of the substrate and a striped electrode 7 is formed on the upper face of the cladding layer 5. Then, a groove 9 is formed at right angles to the striped electrode 7 on the striped electrode 7 by a reactive ion-beam etching technique in a manner to reach the buffer layer 3, resulting in a laser-oscillating area 10 and a light-detecting area 11 on the single substrate 2. The laser oscillating area 5 is composed of the buffer layer 3, the lightemitting layer 4a, the cladding layer 5, and the electrode 7a. The light-detecting area 11 is composed of the buffer layer 3, the light-detecting layer 4b, the cladding layer 5, and the electrode 7b. Laser light produced in the laser-oscillating area 10 is emitted from both facets 10a and 10b of the laser-oscillating area 10. A part of the laser light from the facet 10b is absorbed and detected as electrical signals by the light-detecting layer 4b of the light-detecting area 11. The amount of optical output from the laseroscillating area 10 is monitored by the light-detecting area 11 so as to adjust the amount of current to be injected into the laser-oscillating area 10 to an appropriate level, so that the optical output from the laser-oscillating area 10 can be prevented from varying due to atmospheric temperature, reflected light, etc., and therefore it can be maintained at a fixed level. Such a stabilization of the optical output is significantly important for practical use of semiconductor lasers. However, the production of the light-detecting area 11 is extremely difficult although that the light-detecting area 11 is combined with the laser-oscillating area 10 is effective in improving light-detection characteristics of the light-detecting area 11.
In order to simplify the production process of the light-detecting area 11, the light-detecting area 11 can be integrated with the laser-oscillating area 10 on the single substrate 2 as shown in FIG. 2. However, since the light-emitting layer 4a of the laser-oscillating area 10 and the light-detecting layer 4b of the light-detecting area 11 are epitaxially grown at the same time, the resulting thickness of the light-detecting layer 4b is approximately 0.2 .mu.m which is too thin to attain optimal light-detection characteristics. Moreover, the impurity concentration of the light-detecting layer 4b, which attains the highest detection sensitivity of the light-detecting area 11, is different from that of the light-emitting layer 4a of the laser-oscillating area 10 which attains the optimum laser characteristics. Therefore, the detection sensitivity of the light-detecting area 11 must be enhanced at the sacrifice of the operation ability of the laser-oscillating area 10. On the other hand, laser light is radiated, with spread in the direction of the thickness of the light-emitting layer 4a, from the facet 10b of the laser-oscillating area 10 to the light-detecting area 11, so that only a slight amount of laser light is absorbed by the light-detecting layer 4b of the light-detecting area 11, which causes difficulties in enhancing the detection sensitivity of the light-detecting area 11. Moreover, although such an integration of the light-detecting area 11 with the laser-oscillating area 10 allows the miniaturization of the semiconductor laser apparatus 1, an improvement of the reliability thereof cannot be attained.
In summary, a semiconductor laser apparatus, in which a light-detecting area 11 for minitoring the optical output from a laser-oscillating area 10 so as to maintain the output at a certain level is integrated with the laser-oscillating area 10 on a single substrate 2 into one body, is disadvantageous in that the detection efficiency of the light-detecting area 11 cannot be enhanced.